Most of the description of the present invention has been made using in terms of electrocardiographic and magnetocardiographic data. However, the present invention is not limited to these two types of data but also applies to other physiological and health data, including temperature, respiration, blood pressure, vasomotor activity, physical activity, and body position, that can be measured by non-contact techniques and devices.
Electrocardiography (ECG) is a widely used, simple examination of cardiac electrophysiological system that consists of measuring cardiac electrical potentials. Most ECG systems require a direct contact between the recording electrodes and surface of the skin of a subject. This contact can be further enhanced by using a conducting jel, cleaning and shaving the skin under the electrodes. While these direct-contact methods and devices constitute current gold standard in ECG diagnostics, they are not practical or convenient for long-term, continuous measurements, such as measurements performed over hours or days, weeks or months. The contact systems can be tolerated usually for 24-48 hours. Longer use is problematic, because the electrodes are not convenient for long-term use and may lead to skin irritation, itching, and other skin reactions.
Recently, C J Harland, et. al. described a non-contact electrocardiographic system that can be used for continuous ambulatory monitoring (C J Harland, T D Clark and R J Prance, High resolution ambulatory electrocardiographic monitoring using wrist-mounted electric potential sensors, Meas. Sci. Technol. 14 (2003) 923-928). They describe a newly developed electric potential sensor and its application for the ambulatory monitoring of the human ECG and show that ECG can be acquired using two of these sensors mounted on each wrist. These sensitive and low-noise sensors do not require a real current conducting path in order to operate and can work without making electrical contact to the subject. Furthermore, the same group showed that the sensors can record ECG even at some distance from the skin of a subject separated by several layers of clothing (C J Harland, T D Clark and R J Prance, Electric potential probes-new directions in the remote sensing of the human body Meas. Sci. Technol. 13 (2002) 163-169). In these publications, the authors showed that the method is sensitive to different noises and artifacts and therefore, the signals require filtering. The authors also point out that the recordings were made in an electrically shielded room, which reduces the noise and interference.
Although these authors described potential applications of their system for dynamic analysis of data on a beat-by-beat basis in an ambulatory setting, they did not describe how this system can be used for analysis of longer trends, which may include dozens-to-hundreds-to-thousands of beats during the course of longer-term continuous or serial recordings. The authors do not describe the methods that can be used to prevent or reduce the problems related to the varying environmental conditions (humidity, temperature), varying physical activity, body position, and other confounding physiological and non-physiological factors that can interfere with the ECG signal. Kingsley et. al. describe non-contact high-impedance optical electrodes that can be used for ecg measurements (S. A. Kingsley, A. Sriram, A. Pollick, J. Caldwell, F. Pearce, H. Ding, Physiological monitoring with high-impedance optical electrodes (photrodes), The 23rd Annual Army Science Conference, Dec. 2-5, 2002, Orlando Fla.). The authors also describe applications of their technology for analysis of ECG on a beat-by-beat basis.
Magnetocardiography (MCG) is a well-known method for measuring electromagnetic signals of the heart, and a number of systems for measurements of MCG are known. Kandori et al. (U.S. Pat. No. 6,681,131) discloses an apparatus for measuring bio-magnetic fields that includes a plurality of magnetometers for detecting magnetic fields generated from a live body; a driving circuit for driving the magnetometers; a computer for collecting output signals of the driving circuit in the form of data representing at least one waveform of the magnetic fields generated from the live body and for performing an arithmetic processing on the data representing the waveform of magnetic fields, a display unit, and at least one signal processing circuit for processing output signals of the driving circuit. The computer further performs an arithmetic processing of first-order differentiation on the waveform of the magnetic fields in a z direction with respect to time in order to create an arrow map and a contour map. Tsukamoto (US Patent Application 20040232912) discloses a magnetic field measurement system that allows measurement of an extremely weak magnetic field (such as MCG) by efficiently canceling the external field, in which plurality of sensing magnetometers for measuring a magnetic field signal in a direction perpendicular to the center axis of a cylindrical magnetic shield are arranged in two dimensions on a plane parallel to the center axis and a reference magnetometer for measuring the external field parallel to the center axis as a reference signal on a plane perpendicular to the plane parallel to the center axis. An article entitled “Dynamical Mapping of the Human Cardiomagnetic Field with a Room-temperature, Laser-optical Sensor” by Bison et. al in Optics Express 2003, Vol. 11, pages 904-909 describes a laser-optical magnetocardiographic system, which operates at room temperatures, does not require magnetically-shielded room, and allows measurement of beat-to-beat MCG dynamics. However, all of the above-described systems require highly sensitive sensors and data acquisition system that have significant weight and size. The size of such systems is usually significantly bigger than that of a laptop computer, and therefore are not suitable to be used as a wearable device.
MCG provides essentially the same information as the more widely used electrocardiogram (ECG). The main advantage of MCG is that this method does not require direct attachment of the sensors to the skin of a subject in contrast to ECG measurements. Therefore, the non-contact MCG measurements are more convenient than the ECG measurements and can be used over prolonged time intervals, whereas the duration of ECG measurements is usually limited to a maximum of several days (due to the inconvenience of constantly attached ECG electrodes, which might lead to itching, rash, and other skin reactions when the electrodes are attached for prolonged periods of time). Furthermore, the requirement for a good contact between the ECG electrodes and the skin of a subject often requires shaving and application of special conductive gels. In contrast, non-contact MCG measurements do not require any skin preparation and can be used continuously or repeatedly for any period of time, creating a possibility for monitoring development of slow, gradual changes in the cardiac function over a period of days-weeks-months. Such slow dynamics often characterizes development of chronic cardiac disease, age-related changes, treatment effects, changes caused by physical exercises, and other behavioral, pharmacological, and physiological effects. Furthermore, the convenience of MCG measurements allows an individual (even without a medical background) to perform the measurements by him/herself, so that the device becomes a personal monitoring/diagnostic/registration/screening/check-up system, which records the data collected over prolonged periods into a personal database. Thus, these features make MCG an optimal method for screening of wide populations of people for signs of cardiac abnormalities, monitoring slow, gradual changes in the heart that develop over the course of weeks-months-years that cannot be detected during a one-time examination, examining cardiac functions in the elderly and people at risk or with a history of cardiovascular disease, and monitoring disease development, treatment effects, effects of physical activity, and other pharmacological, behavioral, physiological, and health effects on the heart.
However, MCG has been used rarely by medical professionals, because biomagnetic signals of the heart are relatively small, and their measurements requires sensitive sensors that are expensive and have a relatively large size. In particular, previously known MCG systems are based on optical, laser-optical, or superconducting quantum interference devices (SQUIDs). These devices provide an accurate measurement of MCG, but are relatively large and are not adapted to be miniaturized to a size of a device that can be wearable.
Prior art MCG systems were designed to provide an accurate representation of biomagnetic field distribution in different regions of the heart and body. However, they usually analyze an averaged cardiac complex or a few complexes. There is a need for a wearable device and system for analysis of gradual, slow serial changes in the cardiac beats over different periods of time.